SEDIMENTOLOGICAL CHARACTERISTICS OF FLUVIAL CONGLOMERATES AND SANDSTONES AS INDICATORS OF PALEOGEOGRAPHIC AND PALEOCLIMATIC CONDITIONS, MID-PERMIAN QUANZIJIE LOW-ORDER CYCLES, BOGDA MOUNTAINS, NW CHINA

Tectonic, climatic, and depositional conditions are a major control on the sedimentological characteristics of fluvial sediments. The conglomerates and sandstones of the Quanzijie low-order cycles (QZJ LCs) in Bogda Mountains, NW China, provide clues on how provenance lithology, transport distance, and depositional and paleoclimatic conditions had affected the composition and texture of fluvial deposits. Composition and texture of 1688 gravels in 16 conglomerate beds and 42 sandstones define two petrofacies and five subfacies in Tarlong-Taodonggou area in southern Bogda and Dalongkou area 70 km to the north in northern Bogda. Petrofacies I occurs in Tarlong-Taodonggou and contains dominantly texturally-immature basaltic lithics. Variations in grain shape and paleocurrent direction suggest two sources: Tian Shan volcanic arc to the south and local rift shoulders. Sandstones deposited in ephemeral streams are relatively rich in sedimentary lithics, whereas those in meandering streams are poor. A subtle increase in compositional maturity occurs from the lower-middle to upper parts. Pedogenic features increase in the upper part. The upsection trends conform to increasingly humid conditions in the upper part of the cycle. Petrofacies II occurs in Dalongkou and is dominated by sedimentary lithics, which were likely derived from sedimentary rocks exposed at rift shoulders. They were deposited in meandering streams, and contain abundant pedogenic clay coats. In conclusion, major changes in composition in Tarlong-Taodonggou and Dalongkou suggest provenance lithology as a major control on conglomerate and sandstone petrofacies. Changes in depositional and paleoclimatic conditions and transport distance are secondary. The understanding of the controlling processes and factors is useful to reconstruct the paleogeography of the Turpan-Junggar basin during Middle Permian. The results substantiate the interpretation that the basin was an intracontinental rift basin containing many small grabens and half grabens, one of which is the Tarlong-Taodonggou half graben.